1. Technical Field
The present invention relates to doped fiber amplifiers for utilization in lightwave transmission systems and, more particularly, to pumping configurations for doped fiber amplifiers.
2. Description of the Prior Art
Lightwave transmission systems, in particular undersea and terrestrial loop distribution systems, are beginning to utilize doped fiber amplifier technology to replace the conventional electronic regenerators previously used to provide amplification of lightwave signals traveling long distances. There exists many arrangements in the art for providing such doped fiber amplification. One such arrangement is disclosed in U.S. Pat. No. 5,042,039 issued to Edagawa et al on Aug. 20, 1991. As described therein, an erbium-doped optical fiber is utilized to provide gain to a communication signal by simultaneously passing a "pumping" signal (at an appropriate wavelength) through the erbium-doped medium.
Various arrangements exist in the art for improving the efficiency of such doped fiber amplifiers. One arrangement, capable of providing gain equalization, is disclosed in U.S. Pat. No. 5,050,949 issued to DiGiovanni et al. on Sep. 24, 1991. In this particular arrangement, a multi-state optical fiber amplifier is provided, where each stage is pumped separately and the dopant composition of each erbium-doped fiber section is controlled to provide for gain equalization along the entire communication path. As disclosed an optical isolator is required between the stages to suppress reflection-induced noise.
A need in most lightwave transmission systems is to provide for back-up in case of the failure of a transmitting source. In the case of doped fiber amplifiers, the need often arises to provide a back-up pump laser source. Further, the choice of pump wavelength must often be chosen between a first value of approximately 1.48 .mu.m, which provides maximum gain or 0.98 .mu.m, which provides for low noise. Additionally, many systems exist which require a pump power level greater than that available from a single source. Current arrangements for providing increased pump power may utilize a pair of pump sources and require polarization control of the pump signals such that a polarization combiner may be utilized to launch pump pump signals into the doped fiber section. These and other issues remain of current concern in the doped fiber amplifier technology.